AUTHOR=Martinez Stephane , Kolodny Yuval , Shemesh Eli , Scucchia Federica , Nevo Reinat , Levin-Zaidman Smadar , Paltiel Yossi , Keren Nir , Tchernov Dan , Mass Tali 

TITLE=Energy Sources of the Depth-Generalist Mixotrophic Coral Stylophora pistillata

JOURNAL=Frontiers in Marine Science

VOLUME=Volume 7 - 2020

YEAR=2020

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2020.566663

DOI=10.3389/fmars.2020.566663

ISSN=2296-7745

ABSTRACT=Energy sources of corals, ultimately sunlight and plankton availability, change dramatically from shallow to mesophotic (30-150 m) reefs. Depth-generalist corals, those which occupy both of these two distinct ecosystems, are adapted to cope with such extremely diverse conditions. In this study, we conducted transplantation of the hermatypic coral Stylophora pistillata from 60 m (mesophotic) to 5 m (shallow) reefs. This species harbors different Symbiodiniaceae genera at the two depths. After 18 months we compared symbiont genera composition, structure, and photosynthetic traits, as well as holobiont trophic position and carbon sources of shallow, mesophotic, and transplanted corals. Symbiodiniaceae genera analysis revealed a limited change in genus composition in the transplanted fragments, of less than 20%, from Cladocopium goreaui (dominant in mesophotic colonies) to Symbiodinium microadriaticum (dominant in shallow colonies). This allowed us to determine which differences in corals between the shallow and mesophotic regions stem from hosting different symbiont clade compositions. Calculation of holobiont trophic position based on amino acid δ15N revealed that heterotrophy represents the same proportion of energy sources in both shallow and mesophotic corals, in contrast to the dogma that predation is higher in corals growing in low light conditions. Photosynthesis is the major carbon source to corals growing at both depths, but photosynthetic rate is higher in the shallow reef corals, implicating both higher energy consumption and higher predation rate in the shallow habitat. In the corals transplanted from the deep to shallow reef, we observed extensive photo-acclimation by the Symbiodinacaea cells, including substantial cellular morphological modifications, increased cellular chlorophyll a, and lower antennae to photosystems ratios. In contrast, non-photochemical quenching remains low and does not improve to cope with high light regime of the shallow reef. Furthermore, host acclimation is much slower in these deep-to-shallow transplanted corals with host tissue density and predation levels remaining low, compared to the shallow water corals, even after long-term transplantation (18 months). Our results suggest that while mesophotic reefs could serve as a potential refuge for shallow corals, the transition is complex, as even after a year and a half the acclimation is only partial.